In a drug delivery device, a piston within a cartridge that contains drug may be displaced with respect to the cartridge in the distal direction by a piston rod which moves in the distal direction with respect to the cartridge. Thereby, a dose of drug can be expelled from the cartridge. A drug delivery device is described in US 2007/0123829 A1, for example.
It is often desirable that the actually delivered dose of drug matches the dose which was previously set by a user or which the device was designed to deliver as close as possible, i.e. dose accuracy should be good.